Manufacture of magnesium hydroxide



Patented Jan.` 7, 1941 UNITED STATES MANUFACTURE OF MAGNESIUM HYDROXIDE Neil R. Collins, San Francisco, Calif., assignor to Marine Magnesium Products Corporation, a corporation of Delaware Application February 21, 1938, Serial No. 191,594

1 Claim. (Cl. ,Z3-201) This invention relates generally to the manufacture of magnesium hydroxide from brines containing convertible magnesium salts. More particularly it relates to the manufacture of magnesium hydroxide from sea water or like brines containing iiuorides, where a contaminating iiuoride content in the iinal product isv undesirable and to be minimized.

In the commercial manufacture of magnesium 10 hydroxide and other magnesium products from brines like sea Water, it has been customary to subject the brine to a pre-treatment for removal of certain impurities, after which the brine is reacted with a reagent like lime, to precipitate magnesium hydroxide. In this connection reference can be made to Farnsworth and Montgomery Patent No. 2,021,501, disclosing a commercial process Where it is desired to secure a fast settling magnesium hydroxide precipitateof generally crystalline character, and Chesny Patent No. 2,089,339, disclosing a process also applicable to sea water, and which produces a relatively amorphous magnesium hydroxide of fairly high settling rate.

The type of brine pre-treatment disclosed in the aforesaid patents involves reacting the brine with lime, to cause precipitation of calcium bicarbonate as calcium carbonate. Precipitated calcium carbonate is then removed, thus minimiz- 3 ing calcium contamination in the linal product.

The amount of lime employed in such pre-treatment is sufcient to combine with the bicarbonate ion present, -to effect substantially complete precipitation of the calcium carbonate. There is 5 precipitated together with the calcium carbonate, a small amount of magnesium hydroxide, but this has been considered an undesirable feature because such magnesium hydroxide is removed beim fore the brine is subjected to the iinal precipitating operation. Magnesium hydroxide removed at this point, is at the expense of the yield of magnesium hydroxide, from Athe brine. Thus in practicing processes such as disclosed in Farnsworth et al. 2,021,501 and Chesny 2,089,339, on sea Water, the amount of lime employed in the pretreatment operation has been regulated so that less than about 5% of the available convertible magnesium salts are precipitated. 50 @perating substantially in accordance with Chesny 2,089,339, it has been found that the resulting magnesium hydroxide analysis is from about .0l00 to .01.75% fluorine content, irrespective of purication treatment of the settled magnesium hydroxide. Although filtering or Washing methods, as for example a counterflow washing method such as disclosed in Farnsworth Patent No. 2,021,501 materially reduces certain impurities, like calcium content and sodium chloride, the fluoride content remains with the magnesium hy- Cil treatment stages. Thus by the simple expedient of substantially increasing the amount of lime employed in ythe pre-treatment of the brine, fluoride contamination of the final magnesium hydroxide can be reduced below desired limits, without adding additional steps to the process.

In view of the foregoing it will be evident that the principal object of this invention is to reduce 4 iiuoride contamination in .magnesium hydroxide precipitated from brines, like sea Water.

Referring to the drawing, Fig. 1 4is a iiow sheet, illustrating diagrammatically the carrying out of the present process.

Fig. 2 is a flow sheet illustrating a modification of the general process illustrated in Fig. 1.

Fig. 3 is a diagrammatic view illustrating a further possible modification of the process.

Referring rst to the fioW sheet of Fig. l, brine such as sea water, containing convertible mago nesium salts, calcium bicarbonate and fluorides, is

subjected to a rpre-treatment operation l0. 1n this pre-treatment a measured amount of lime is introduced into the brine, preferably in the form of a lime slurry. The amount of lime is such as to not only satisfy the bicarbonate ion present, Ithus causing precipita-tion of calcium carbonate, but to also cause precipitation of a substantial percentage of the convertible magnesium salts, thus precipitating relatively amorphous magnesium hydroxide which can be readily retained di-spersed Within the body of the brine.

The next stage l l involves a period of retention of the magnesium hydroxide dispersed throughout and in intimate contact with the brine, whereby there is an absorption or adsorption of flucrides present, by the particles of magnesium hydroxide. Retention also serves to complete the reaction with the bicarbonate ion, to complete precipitation of calcium carbonate. Following this retention, which may continue for an effective period of two to six hours, solids are removed from the brine to produce a clarified brine for the nal precipitating operation. Removal o f solids can be conveniently carried out by vsand ltration, or settlement .and filtration. Solids removed include the precipitated calcium carbonate and magnesium hydroxide, together with other solids such as coagulated organic material. Removal of organic material may be aided by the use of Well known coagulants, such as salts of iron or aluminum.

The next stage I2 is the main precipitating operation, to precipitate 4the remainder o! the 6 convertible magnesium salts as magnesium hydroxide. This precipitating operation can be carried out -as disclosed in Farnsworth et al. 2,021,501, or Chesny 2,089,339, depending upon the character of .the precipitate desired. Assuming 10 that it is desired to obtain a relatively amorphous form ot precipitate, -the process of Chesny 2,089,339, is preferred. Thus the precipitant employed is highly reactive lime slurry, which is commingled with the lime slurry in such a manner as to form a relatively rapid settling precipitate. The precipitatedmagnesium hydroxide is permitted to settle through the mother liquor, and is removed from the lower part of the precipitating tank, as a concentrated slurry.

The slurry of precipitated magnesium hydroxide obtained by operation I2, can be subjected to a further purification operation I3. For this purpose it is desirable to make use of the purifying apparatus and method disclosed in Farnsworth 2,021,501, wherein soluble impurities including sodium chloride, and some remaining calcium content, are removed by leaching with fresh water.

The washing or purifying operation I3 results in the iinal purified magnesium hydroxide slurry,

which can be treated to form other magnesium products, such as powdered magnesium hydroxide, magnesium carbonates, `and magnesium oxides.

As an example of commercial operations in accordance with the present process, about 3.5 lbs.

of calcium oxide, slaked to form a slurry, has been used for the pre-treatment of each 1000 gallons of brine. The brine was San Francisco Bay water, con-taining approximately 2.75% sodium chloride content, and having a specic gravity of 1.025. The effective retention time during pretreatment was about 2 hours. Upon analysis of the final precipitated magnesium hydroxide, (on a dry basis), it was found lto contain from .002 to .0025% fiuorine.

In order to properly appreciate the results of the present process, as distinguished from prior practice, pre-treatment of the same brine with approximately 1.5 lbs. of calcium oxide per 1000 gallons of brine, in a pretreatment opera-tion intended only for the removal of calcium bicarbonate and organic solids, will afford a nal precipitated magnesium hydroxide having about .0125 to .015% iluorine content.

In practice I prefer to so adjust the amount of lime used in the pretreatment operation, that from about 10 to 25% of the available convertible magnesium salts, are precipitated as magnesium hydroxide. From about 12 to 20% is an optimum range, affording only a slight percentage of fiuorine in the final magnesium hydroxide, without too great a sacrice of the available magnesium hydroxide in the original brine. In the foregoing commercial example of the present process the amount of lime employed is sufcient to precipitate approximately 15% of the convertible magnesium salts, in the pretreatment operation. In the above example of prior practice about 5% of the magnesium salts were precipitated during pretreatment. y

Fig. 2 illustrates more specifically a procedure which can be followed in the pretreatment and precipitation of the brine. The source of brine is supplied to the storage tank I4, into which the pretreatment lime is also introduced. Within this tank liquid is subjected to some agitation, to

insure proper reaction and to maintain the precipitated magnesium hydroxide in suspension. Some heavier solids, including heavy foreign sol- 5 ids and organic material, settle out in this tank, and can be removed from the bottom of the tank as a sludge.

From storage tank I4, the brine is supplied to the treatment tank I5, where it is intermixed l0 with a suitable coagulant, such as ferrous sulphate. While in this tank there is further opportunity for intimate contact between the precipitated magnesium hydroxide and the brine, to enable absorption, or adsorption of fiuorides, i5 by the particles of magnesium hydroxide. Some sludge is also formed in this tank, and can be removed from the lower end of the tank, to avoid subsequent removal in the filtration operation. The overow from tank I5 is supplied to the sand 20 illter I6, where remaining solids are removed, including remaining precipitated calcium carbonate, and magnesium hydroxide. The clarified brine from the sand lters I6, is then supplied to the reaction container I1, which is po- 25 sitioned Within the main precipitating tank I8. Lime slurry is also supplied to the reaction box I1, and by virtue of the intermixing and commingling within box I1, magnesium hydroxide is precipitated. The amount of lime employed is 30 suiiicient to react with all or a substantial portion of the remaining convertible magnesium salts. From box I1, liquid together with flocs o! precipitated` magnesium hydroxide flow outwardly below the surface of the mother liquor, 35 and the magnesium hydroxide settles to the lower portion of the tank, where it is removed as a thickened slurry.

Fig. 3 illustrates a type of equipment which canI be substituted in place of the retention yor 40 storage tank I4, of Fig. 2. In this instance the tank 20 is provided with slowly rotating varies 2i. The brine to 'be treated is introduced into the inow launder 22, together with the pretreatment lime. The slowly rotating vanes 2| 45 within the tank 20 tend in eii'ect to activate precipitation of calcium carbonate, and absorption of fiuorides by the magnesium hydroxide, thus effecting a saving in retention time and size of the equipment. 'I'he effluent for tank 20 can be 50 supplied to tank I5 of Fig. 2.

I claim:

In a process for the manufacture of magnesium hydroxide from sea water, the steps of reacting normal sea water with lime to precipitate 55 calcium bicarbonate as calcium carbonate and to react with a substantial but minor part of the magnesium salts to precipitate magnesium hydroxide, the amount of lime employed being sufficient to precipitate the calcium bicarbonate 60 as calcium carbonate and to afford an excess sufficient to precipitate from about 12 to 20% of the convertible magnesium salts present, maintaining the precipitated magnesium hydroxide dispersed throughout the body of the sea water with G5 a period of retention to cause absorption of the fiuorides present, removing precipitated calcium carbonate and magnesium hydroxide from the brine, and then reacting the brine with lime to precipitate the remaining magnesium salts as 70 magnesium hydroxide.

NEIL R. COLLINS. 

